1. Field of the Invention
This invention relates to supercritical fluid (SCF) process reactors used in the deposition of metal and dielectric thin films on substrates, such as semiconductor substrates. More specifically, the present invention relates to heaters used in SCF process reactors to heat the substrate during processing.
2. Description of Related Art
Many cleaning and deposition processes, including chemical vapor deposition (CVD), physical vapor deposition (PVD) and supercritical fluid-based processes require heating the substrate to initiate the desired deposit. In conventional low-pressure CVD and PVD processes the substrate is often heated by placing it in direct contact with a relatively thin resistively heated metallic or ceramic plate forming part of the reactor wall. The substrate is heated with a resistive heater placed on the backside of the thin plate. While this design is effective in the low-pressure environment of conventional CVD and PVD reactors, it has a number of shortcomings in the high-pressure environment of SCF processes.
To safely contain the high-pressure (200 to 300 atmospheres) supercritical fluid used in SCF processes, the walls of an SCF reactor are substantially thicker than those used in a CVD or PVD reactor. A wall thickness of greater than 4 inches (10 cm) is commonly used. Using a conventional CVD or PVD reactor heater design in an SCF reactor would require heating the substrate through a very thick wall that must be capable of withstanding the high-pressure sustained by the SCF reactor walls; not the relatively low-pressure encountered in a CVD or PVD reactor.
Because the walls of an SCF reactor are normally constructed of a relatively low thermal conductivity material, such as stainless steel or a nickel-molybdenumchromium alloy, a resistive-element heater attached to the backside of a thick wall of such material would have a thermal latency of at least many tens of minutes, making wafer temperature control very difficult and inefficient.
A related problem is that the thick wall would act as a large heat reservoir and would transfer heat to the SCF reactor walls, nearby components and the supercritical fluid. Such wall heating could result in unwanted deposition on the walls or chemical reaction in the supercritical fluid. Also, the power that would be required to heat such a massive plate would be much greater than is required with conventional thin-wall CVD or PVD heater designs.
In a typical CVD or PVD process chamber the chamber walls are effectively thermally isolated from the substrate and from the heater by the low process pressure (near vacuum) in the chamber. However, in an SCF reactor the supercritical fluid has a very high thermal conductivity compared to that of the CVD or PVD vacuum environment. Heat is rapidly transferred through the supercritical fluid medium to all reactor walls and surfaces. Unless this transfer of heat is controlled, deposition will occur on unwanted surfaces and power requirements for the heater will be greatly increased.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus for heating a substrate in a supercritical fluid reactor which avoids the necessity to heat a large wall mass and prevents excess heat loss to the wall and/or SCF which would increase power requirements and result in undesired deposition on the wall or internal components.
It is another object of the present invention to provide a method and apparatus for heating a substrate in a supercritical fluid reactor that has improved temperature control and fast response as compared to existing heater designs.
A further object of the invention is to provide a method and apparatus for heating a substrate in a supercritical fluid reactor that uses the supercritical fluid to maximize heat transfer to the substrate.
It is yet another object of the present invention to provide a method and apparatus for heating a substrate in a supercritical fluid reactor that does not require the heater to be a structural component of the SCF reactor wall.
Still another object of the present invention is to provide a method and apparatus for heating a substrate in a supercritical fluid reactor in which all heater components are in an isobaric environment.
Yet another object of the present invention is to provide a method and apparatus for heating a substrate in a supercritical fluid reactor in which the edges and backside of the substrate are protected from undesired deposition.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in the art, are achieved in the present invention, which is directed to a heater for heating a substrate in a supercritical fluid reactor. The heater includes a clamp, a heater body having a heater chamber, a heating element in the heater chamber and an insulator in the heater chamber insulating the heating element from the walls of the heater chamber. The clamp and the heater body are relatively movable to clamp the substrate between them and form an isolated heater chamber.
When the substrate is disk-shaped it is preferred for the clamp to be a clamp ring and clamp the perimeter of the substrate against the perimeter of the heater chamber. The clamp ring includes a clamp ring seal that cooperates with a heater body seal and seals against the outer edge of the substrate.
In the preferred design, the heater body is integral to the wall of the supercritical fluid reactor. In the most highly preferred embodiment of the invention, the heating element is spaced apart from the insulator, from the back of the substrate and from the walls of the heater chamber and supercritical fluid is used to transfer heat from the heating element to the back of the substrate.
The heater is designed to maintain an equal pressure on both sides of the substrate.
The insulator may be constructed of pyrolytic graphite which has excellent properties for preventing heat from escaping the heater chamber while ensuring uniform heating of the substrate.
The invention is also directed to a method of heating a substrate in a supercritical fluid reactor including the steps of:
providing a heater chamber open to an interior of the supercritical fluid reactor, the heater chamber including a heating element insulated from a wall of the supercritical fluid reactor;
positioning the substrate between the heater chamber and the interior of the supercritical fluid reactor
filling the supercritical fluid reactor and the heater chamber with supercritical fluid;
sealing a perimeter of the substrate to a perimeter of the heater chamber to seal supercritical fluid in the heater chamber; and
heating the substrate with the heating element.
In another aspect of the invention the method includes the steps of:
providing a heater for heating the substrate, the heater including:
a heater body defining a heater chamber,
a clamp adapted to clamp a perimeter of the substrate to a perimeter of the heater chamber,
a heating element having a front side and a backside, the heating element being mounted within the heater chamber, and
an insulator mounted within the heater chamber to insulate the heating element from the heater body;
positioning the substrate between the clamp and the heater body; filling the supercritical fluid reactor and the heater chamber with supercritical fluid;
pressurizing the supercritical fluid in the supercritical fluid reactor and the heater chamber to a desired pressure;
clamping the substrate between the clamp and the heater body after the supercritical reactor is pressurized to form a sealed heater chamber containing supercritical fluid at substantially the same desired pressure as the pressure of the supercritical fluid in the supercritical fluid reactor; and
heating the substrate with the heating element.